\({\mathbf {A}}(u)\)  is a Hermitian matrix.
Matrix \({\mathbf {U}}\)  has an expression through the so called Magnus expansion [1]},
\({\mathbf {U}}(u) = \exp \lbrace -i t_f \lambda _0(u) {\mathbf {I}}- \mbox{$\Xi $}(u) \rbrace , \;\; \mbox{$\Xi $}(u) = \sum _{k=1}^\infty i^k \mbox{$\Xi $}_k(u),\) 
